Open System Settings -> Appearance, and select Windows, and take a look on the Button tab. Here you can customise the buttons in the title bar. '?', '_' and the square represent the What's this?, minimise, and maximise buttons. 'H' is a spacer. One or more can be added as desired. Your version may also have a circle - this is the ring of a radio button, the setting for showing windows in all desktops.
